Consider the reaction H2(g) + Br2(l) → 2 HBr(g). (b) Without doing a calculation, predict whether your estimate in part (a) is more negative or less negative than the true reaction enthalpy.

Verified step by step guidance

Step 1: Understand the reaction: The reaction given is H2(g) + Br2(l) → 2 HBr(g). This is a synthesis reaction where hydrogen gas reacts with liquid bromine to form hydrogen bromide gas.

Step 2: Consider the states of matter: Note that H2 is a gas, Br2 is a liquid, and HBr is a gas. The phase change from liquid to gas for Br2 will require energy input, which affects the enthalpy.

Step 3: Analyze bond energies: Breaking bonds requires energy, while forming bonds releases energy. In this reaction, the H-H and Br-Br bonds are broken, and new H-Br bonds are formed.

Step 4: Compare bond energies: Typically, the energy required to break the H-H and Br-Br bonds is less than the energy released when forming two H-Br bonds, suggesting an exothermic reaction.

Step 5: Predict the enthalpy change: Since the reaction is likely exothermic, the true reaction enthalpy is negative. If your estimate in part (a) did not fully account for the energy released in forming H-Br bonds, it might be less negative than the true reaction enthalpy.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy of Reaction

The enthalpy of reaction, or reaction enthalpy, is the heat change that occurs during a chemical reaction at constant pressure. It is a key concept in thermodynamics, indicating whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). Understanding this helps predict the energy changes associated with the formation of products from reactants.

Standard Enthalpy of Formation

The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. This concept is crucial for estimating reaction enthalpies, as it allows for the calculation of the overall energy change by using tabulated values for the reactants and products involved in the reaction.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to complete the reaction. This principle allows chemists to calculate the enthalpy change of a reaction indirectly by summing the enthalpy changes of individual steps, which is particularly useful when direct measurement is difficult or impossible.

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Hess's Law

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